A commercially available oxygen mask is customarily provided with an inflatable pneumatic head harness, this form of harness being generally illustrated in U.S. Pat. No. 3,599,636 issued Aug. 17, 1971. When this unit is installed in an aircraft, it is connected directly to the aircraft main oxygen system and, prior to use, is conventionally stored in a storage box. When the mask assembly, that is to say the inflatable head harness and the face mask, is to be used, it is removed from the storage box. Before the device can be placed over the operator's head it is necessary to inflate the harness. In addition, there is also a requirement that the oxygen system not only almost instantaneously inflate the harness but supply breathing gases within a fraction of a second after removal from the storage box. Thus, in the past, if the aircraft did not have an oxygen system, these devices could not be used. A breathing device is now required for pilots when flying at high altitude in the event of a rapid decompression. Also, it has been found that in some situations the aircraft oxygen system is not as reliable as chemical oxygen generators of the type customarily referred to as chlorate candles. Therefore, it is desirable that a unit of the type shown in the aforementioned U.S. Pat. No. 3,599,636 be provided which can be utilized with a chemical oxygen generator such as a chlorate candle. However, such a mask assembly cannot be connected directly to a chlorate candle as it takes a few seconds before the volume of the gas generated by the chlorate candle is sufficient. Thus, should a rapid decompression occur, the wearer of the mask would not be able to don the head harness and to start breathing as quickly as required, especially if the situation is such as to maintain control of the aircraft while donning the unit.